Microfabrication of Nanoporous Gold Patterns for Cell-material Interaction Studies

Summary

We report on techniques to micropattern nanoporous gold thin films via stencil printing and photolithography, as well as methods to culture cells on the microfabricated patterns. In addition, we describe image analysis methods to characterize morphology of the material and the cultured cells using scanning electron and fluorescence microscopy techniques.

Abstract

Nanostructured materials with feature sizes in tens of nanometers have enhanced the performance of several technologies, including fuel cells, biosensors, biomedical device coatings, and drug delivery tools. Nanoporous gold (np-Au), produced by a nano-scale self-assembly process, is a relatively new material that exhibits large effective surface area, high electrical conductivity, and catalytic activity. These properties have made np-Au an attractive material to scientific community. Most studies on np-Au employ macro-scale specimens and focus on fundamental science of the material and its catalytic and sensor applications. The macro-scale specimens limit np-Au's potential in miniaturized systems, including biomedical devices. In order to address these issues, we initially describe two different methods to micropattern np-Au thin films on rigid substrates. The first method employs manually-produced stencil masks for creating millimeter-scale np-Au patterns, while the second method uses lift-off photolithography to pattern sub-millimeter-scale patterns. As the np-Au thin films are obtained by sputter-deposition process, they are compatible with conventional microfabrication techniques, thereby amenable to facile integration into microsystems. These systems include electrically-addressable biosensor platforms that benefit from high effective surface area, electrical conductivity, and gold-thiol-based surface bioconjugation. We describe cell culture, immunostaining, and image processing techniques to quantify np-Au's interaction with mammalian cells, which is an important performance parameter for some biosensors. We expect that the techniques illustrated here will assist the integration of np-Au in platforms at various length-scales and in numerous applications, including biosensors, energy storage systems, and catalysts.

Introduction

Materials with nano-scale features have shown promise in enhancing various applications, including fuel cells¹, sensors^2,3, and biomedical devices^4,5. A relatively new material is nanoporous gold (np-Au), which is produced by a nano-scale self-assembly process. The precursor to np-Au is a gold alloy that most commonly consists of silver at 60% to 80% by atomic percentage. Briefly, the characteristic open-pore nanostructure is the result of rearrangement of gold atoms in clusters as silver is dissolved by a strong acid (e.g. nitric acid 70%) or under an electrochemical potential^6-8. Np-Au benefits from seve....

Protocol

1. Nanoporous Gold Fabrication

1. Clean substrates in Piranha solution
   1. Add 25 ml hydrogen peroxide (30%) to 100 ml sulfuric acid (96%) in a crystallization dish and heat the mixture to 65 °C on a hotplate. CAUTION: The liquids are extremely corrosive and must be handled with care. The spent solution should not be stored in a sealed container, as it may explode.
   2. Place 1-inch by 3-inch microscope slides into the mixture using acid-resistant forceps and clean them for 10 min. Use a porcelain immunostaining boat for batch-cleaning small coverslips. Treat small coverslips with air plasma at 10 W for 30 sec prior to immersion into the sol....

Results

Figure 1 outlines the major procedural steps, including creating the np-Au patterns, culturing cells, quantifying the nanostructure, and characterizing cell morphologies. The elastomer stencil shown in Figure 2a (top) is used for creating the np-Au patterns shown in the images underneath. Figure 2b is a photograph of the porcelain boat for batch processing specimens. Figure 2c displays the color change of the deposited metal patterns before and afte.......

Discussion

We demonstrate two different techniques to micropattern np-Au films for expanding use of these films in microsystems and biological studies. Sputter-coating gold and silver is a versatile method to create np-Au patterns, as sputtering is compatible with conventional microfabrication processes and the alloy composition and thickness can be easily controlled by varying the individual sputtering gun powers (for gold and silver targets) and the deposition time respectively. Typical np-Au film thicknesses range from 200 nm to.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Gold target	Lesker	EJTAUXX403A2	Precursor to alloy for producing np-Au
Chrome target	Lesker	EJTCRXX353A2	Adhesive layer
Silver target	Lesker	EJTAGXX403A2	Precursor to alloy for producing np-Au
Porcelain boat	Thomas Scientific	8542E40	Used for processing small samples
Nitric acid	Sigma-Aldrich	43873	Used at 70% for dealloying
Sulfuric acid	J.T Baker	7664-93-9	Used at 96% for piranha cleaning
Hydrogen peroxide	J.T Baker	7722-84-1	Used at 30% for piranha cleaning
Biopsy punches	Ted Pella	150xx	Available in several sizes
Silicone elastomer sheets	Rogers Corporation	HT 6240	Available in several thicknesses
Hexamethyldisilazane	Sigma-Aldrich	440191-100ML	Used as adhesion promoter for positive resist
Microposit MF CD26	Shipley	38490	Positive photoresist developer
PRS 3000	J.T Baker	JT6403-5	Positive photoresist stripper
Circular glass coverslips (12 mm)	Ted Pella	26023	Used as substrate for metal patterns and cell culture
Glass slides (1 x 3 inch)	Ted Pella	26007	Used as substrate for metal patterns
Kapton polyimide tape	VWR	82030-950	Used for securing elastomer
Transparency masks	Output City		Used in photolithography http://www.outputcity.com/
Plasma cleaner	Harrick Plasma	PDC-32G	Used for activating glass surfaces
Sputtering machine	Kurt J. Lesker	LAB18	Used for depositing metals